I have a question. Is the door you are trying to make open a separate mesh? You say you are
"trying to create a morph target in an existing mesh of a room with a door that doesn't open" If the door isn't a separate object group than none of the advice they have given you will work. You would have to cut the door out of the room mesh in order to open it. Sorry if this is already obvious to you. Just trying to cover all the basses in order to solve the problem.